CHAPTER III. 



STRUCTURE OF THE ELEMENTARY TISSUES. 



IN this chapter the leading characters and chief modifications of two 

 great groups of tissues the Epithelial and Connective will be briefly de- 

 scribed; while the Nervous and Muscular, together with several other 

 more highly specialized tissues, will be appropriately considered in the 

 chapters treating of their physiology. 



EPITHELIUM. 



* 



Epithelium is composed of cells of various shapes held together by a 

 small quantity of cementing intercellular substance. 



Epithelium clothes the whole exterior surface of the body, forming 

 the epidermis with its appendages nails and hairs; becoming continuous 

 at the chief orifices of the body nose, mouth, anus, and urethra with 

 the epithelium which lines the whole length of the alimentary and genito- 

 urinary tracts, together with the ducts of their various glands. Epi- 

 thelium also lines the cavities of the brain, and the central canal of the 

 spinal cord, the serous and synovial membranes, and the interior of all 

 blood-vessels and lymphatics. 



The cells composing it may be arranged in either one or more layers, 

 and thus it may be subdivided into (a) Simple and (b) Stratified or 

 laminated Epithelium. A simple epithelium, for example, lines the 

 whole intestinal mucous membrane from the stomach to the anus: the 

 epidermis on the other hand is laminated throughout its entire extent. 



Epithelial cells possess an intracellular and an intranuclear network 

 (p. 10). They are held together by a clear, albuminous, cement sub- 

 stance. The viscid semi-fluid consistency both of cells and intercellular 

 substance permits such changes of shape and arrangement in the individ- 

 ual cells as are necessary if the epithelium is to maintain its integrity in 

 organs the area of whose free surface is so constantly changing, as the 

 stomach, lungs, etc. Thus, if there be but a single layer of cells, as in 

 the epithelium lining the air vesicles of the lungs, the stretching of this 

 membrane causes such a thinning out of the cells that they change their 

 shape from spheroidal or short columnar, to squamous, and vice versa, 

 when the membrane shrinks. 



